FIFTH, OR AGASSICERAN BRANCH. 195 



the prolonged existence of the goniatitic proportions, which are generally con- 

 fined to naspionic stages of growth in other Ammonitinse. They are also very 

 similar in some of the nealogic and adult stages, both in form and characteristics, 

 if we except the sutures, to Psiloceras. 1 The sutures acquire the deep arietian 

 abdominal lobe quite early in life. The sutures of the higher species, Agassi- 

 ceras Scipionianum, are very similar to those of Coroniceras. The affinities of 

 Agas. Icevigatum and Ast. obtusum, and also those with Psiloceras, have been fully 

 discussed elsewhere. 2 In the clinologic stage the abdomen becomes more acute, 

 and the sides smooth. Neumayr has lately redescribed this genus in part 

 under the name of Cymbites. 3 



Agassiceras laevigatum, Hyatt. 



Plate VIII. Fig. 9-14. Sum m. PI. XIII. Fig. 1. 



Amm. Icevigatus, Sow., Min. Conch., pi. 570, fig. 3. 



Amm. Icevigatus, Opp., Die Juraf., p. 81. 



Amm. abnorme, Hauer, Unsymm. Aram. Hierl.-Schicli., Sitz. Akad. Wien, XIII. pi. i. fig. 11-17. 



Cymbites globosus, Geyer, Ceph. Hierl.-Schich., pi. iii. fig. 26. 



Localities. — Lyme Regis, Seniur, 



This species has an exceedingly immature or naepionic form. It seems 

 frequently to complete its growth in five whorls. The aperture has a simple 

 pointed rostrum, the lateral edges slightly flaring, with a broad shallow constric- 

 tion, PL VIII. Fig. 12, very similar to the aperture of Oppel's type of Amm. pla- 

 norbis} The living chamber is about one half a volution in length. 



Var. A, Plate VIII. Fig. 11. This is smooth during four volutions, and the 

 pilae, if present, are developed only on the last whorl. The whorls are more 

 compressed than in other varieties, and the umbilicus not so deep. 



Var. B, Plate VIII. Fig. 9, is smooth only during the first three or three 

 and a half whorls, and then has immature folds like those of var. plicalum of 

 Psil. planorbe. The younger whorls are wider than in other varieties, and the 

 umbilicus deeper. 



Var. C has the pilae much more distinct, but the period at which they appear 

 is the same as in the preceding variety. The pilae are apt to cross the abdomen, 

 forming slight ridges. This, like all other characteristics, is found more or less 

 also in other varieties. 



Var. D, Plate VIII. Fig. 10-12, is founded on the presence of a faintly 

 defined siphonal ridge. This includes members of other varieties, regardless of 

 the time at which the pilaa are developed, their greater or less prominence, and 



1 A paper by E. Haug, " Ueber Polymorphidae," Neu. Jahrb. Mineral., 1887, II. p. 92, gives an in- 

 teresting account of this genus, in which he substantially agrees with our descriptions, but insists upon the 

 keelless character of the whorls. If we are correct, this is an adventitious character common enough in 

 dwarfed forms, or arrested development in some individuals or in the varieties of the same species, but a 

 variable within the species, and not a generic character. 



2 See pages 65, 66. 



8 Jahrb. Geol. Reichsans., XXVIII., 187S, p. 64, note. 



4 We have studied Oppel's type of this species, and found that Oppel's figure gives the lateral curves 

 of the aperture in an exaggerated form, and the constriction too deep. The real aperture is therefore more 

 like that of Icevigatum than it appears to be in his figure. 



